If i have a huge surplus of FM's and a reasonable amount of cash (usually late game Romani, Maks, AS etc) i'll use spare FM's to roleplay stuff - like build a fort on/near choke points or resources and stick a low-influence or useless character and RP that they've been 'banished' to some backwater or fallen out of favour etc. Usually only happens when i have too many FM's or generals.

Oh and as mentioned before, i'll usually simulate reforms for the AS and Makedonia by adding units to the roster as i defeat certain factions or have prolonged wars with them